A cold chamber type disc-casting machine in which a molten metal material is injected/filled into a cavity of a mold through an injection sleeve so as to obtain a product has been known well. In the die-casting machine, a metal material (for example, an Al alloy, an Mg alloy, etc.) melted in a melting furnace is measured and scoped by ladle every one shot. The scooped molten metal (molten metal material) is supplied into the injection sleeve. The supplied molten metal is injected/filled into the cavity of the mold by the forward movement of an injection plunger.
A process of casting with a die-casting machine generally includes an injection/filling step and a pressure intensifying step following the injection/filling step. The injection/filling step has a low-speed injection step and a high-speed injection step. A higher injection speed than that in plastic injection molding is required in the high-speed injection step. Generally in the background art, a hydraulic driving source has been used as a driving source for injection/pressure intensifying. Performance of rapid rising to a high speed is required in die-casting machines. When a small-size electric servo motor is used as a driving source for injection/pressure intensifying, the accelerating performance of the small-size electric servo motor cannot satisfy the required performance. However, when a large-size and high-power electric servo motor is used, the cost increases on a large scale.
However, hydraulic die-casting machines may cause a taint damage. Recently there is a growing demand for a clean electric die-casting machine. Such an electric die-casting machine is being developed. A die-casting machine disclosed in Japanese Patent No. 3247086 (Patent Document 1) is an example of a background-art die-casting machine using an electric servo motor as a driving source for injection/pressure intensifying. In the die-casting machine according to Patent Document 1, a fly-wheel unit for accumulating rotational energy is provided. The fly-wheel unit is driven to rotate by an injection electric servo motor before the operation of injection/pressure intensifying. Thus, the rotational energy is accumulated in the fly-wheel unit, and the power from the fly-wheel unit is supplied (added) to the driving power of the injection electric servo motor at an early stage of a high-speed injection step of an injection/filling step and at an early stage of a pressure intensifying step (pressure intensifying/holding step).
FIG. 11 is a chart showing the state of the injection/filling step and the pressure intensifying step (pressure intensifying/holding step) in the die-casting machine disclosed in Patent Document 1. In FIG. 11, the abscissa designates time, and the ordinate designates speed and pressure. As shown in FIG. 11, time of adding power from the fly-wheel unit is provided at an early stage of the high-speed injection step and at an early stage of the pressure intensifying step. Thus, the time of rising to a high injection speed is shortened, and the time of rising to intensified pressure is shortened.
In the background art, as shown in FIG. 11, the injection/filling step is definitely separated into a low-speed injection step and a high-speed injection step, and a speed in a boundary M between the low-speed injection step and the high-speed injection step is set as a broken-line characteristic. The boundary M (timing of termination of the low-speed injection step) between the low-speed injection step and the high-speed injection step corresponds to the timing when degassing has been completed and the head of molten metal has just reached a gate of a mold. The speed (injection plunger speed) is designed to rise rapidly on and after this timing.
Patent Document 1: Japanese Patent No. 3247086
In the background art, as described above, the injection/filling step is definitely separated into a low-speed injection step and a high-speed injection step, and the speed in the boundary M between the low-speed injection step and the high-speed injection step is set as a broken-line characteristic. In order to shorten the time of rising to a high injection speed, the technique disclosed in Patent Document 1 uses the rotational energy accumulated in the fly-wheel unit. However, an ON-OFF controllable clutch is required to use the rotational energy accumulated in the fly-wheel unit. Thus, the transient response in rising to the high speed is slowed by the time of ON operation of the clutch (the response delay time of the clutch). In addition, first of all, there is an obvious limit in improvement of the transient response of servo control because the speed is made to rise suddenly from a constant low speed to a high speed. That is, when the electric servo motor is controlled to be accelerated in the stage where molten metal begins to be injected/filled into the cavity of the mold, there is an obvious limit in shortening the time to reach the high speed. Further, the speed in the boundary M (the timing of termination of the low-speed injection step) between the low-speed injection step and the high-speed injection step is set at a value about 0.7 m/sec or lower in the background art. A specified time is required to rise from such a speed to a high speed.
Of die-casting machines using a hydraulic driving source as a driving source for injection/pressure intensifying, as shown in FIG. 12, there has been known a machine in which constant acceleration control is applied to a low-speed injection step of an injection/filling step by use of a hydraulic servo valve having a high-speed response so that the speed in the low-speed injection step can be increased gradually. Even in the control shown in FIG. 12, the speed in a boundary N between the low-speed injection step and a high-speed injection step is set as a broken-line characteristic. In addition, the boundary N (the timing of termination of the low-speed injection step) between the low-speed injection step and the high-speed injection step is set at a value about 0.7 m/sec or lower. Even in the hydraulic die-casting machine, it cannot be denied that there may be a problem similar to the aforementioned one though there is a difference in degrees.